DON'T PANIC!!
This is only preparation for a future change and would only be the first of the two lists here which can be done while still using GTK3.

All changes should be in platform/guigtk.cpp

There is no hurry to use GTK4 but some day it will be necessary and we might as well know our status. It shouldn't be hard since we don't use many platform UI features. The list is here so we can check them off when they are done or confirmed as non-issues for SolveSpace - there is more detail at the link above.

• Do not use deprecated symbols
• Enable diagnostic warnings
• Do not use widget style properties
• Review your window creation flags
• Stop using direct access to GdkEvent structs
• Stop using gdk_pointer_warp()
• Stop using non-RGBA visuals
• Stop using GtkBox padding, fill and expand child properties
• Stop using the state argument of GtkStyleContext getters
• Stop using gdk_pixbuf_get_from_window() and gdk_cairo_set_source_surface()
• Stop using GtkWidget event signals
• Set a proper application ID
• Stop using gtk_main() and related APIs
• Reduce the use of gtk_widget_destroy()
• Reduce the use of generic container APIs</title>